1. Technical Field
The present invention relates to the field of quality control in manufacturing processes.
2. Discussion of Related Art
In manufacturing processes, quality control systems and procedures are used to identify variations in the process between the manufacture of one item and another in order to ensure consistency. Sometimes such variations are only apparent in the finished product or may not be apparent until after many sub-standard products have been produced. Semiconductor components, such as integrated circuits, may be manufactured in the form of a wafer containing many components. Semiconductor wafers, e.g. silicon wafers, may be manufactured in a series of layers each consisting of a pattern that must be accurately positioned with respect to patterns in the adjacent layers. The control of this positioning is called overlay control. In some semiconductor and lithography manufacturing processes, a metrology target is provided on the wafer for use in ensuring pattern alignment. The target may take the form of a set of cells, for example a 2×2 array of rectangular or square cells, two for measuring overlay in the X direction and two for measuring overlay in the Y direction. The target may include a diffraction grating. For example, each cell in a target may consist of a diffraction grating. A target may consist of a set of patterns, where each pattern may be printed on a different layer and may be oriented such as to provide measurement in different directions, typically X and Y.
In any manufacturing process, process variations may occur which may be undesirable. To take the example of lithographic processes, overlay measurement, e.g. measurement of pattern alignment, may not reveal a process variation that has occurred, such as a variation in thickness of an applied layer of material. In some instances a process variation may lead to incorrect overlay measurement. Therefore there is a need for better understanding of process variations and their effects.